Preparation of microporous filamentous articles



May 5, 1964 .1 -r. EDMoNDs, JR., ETAL 3,132,194

PREPARATION oF MIcRoPoRous FILAMENToUs ARTICLES Filed April 9, 1962 United States Patent O This invention relates broadly to microporous filaments or liber-s yand to lamentous articles formed therefrom. ln accordance with one aspect, this invention relates to VVa method for the preparation of mier-operons filaments or fibers. In another aspect, this invention relates to novel microporous lilaments or bers and articles formed therefrom. l

In recent years, various processes have been developed for the production of polymers of l-olefins, which polymers have Ifo-und Wide utility in the production of various molded items including tilamen-ts or libere and filamentous articles. Although iilaments or fibers formed from 1- volefin polymers have :found Wide utility, the industry is constantly seeking new uses las well as Ways ot improving on the properties of these iil-aments. Filaments formed from high density, solid olefin polymers, especially polyethylene, have excellent tensile properties, chemical resistance and durability. lVlarinek ropes and various wear resistant fabrics have been Woven or knitted from these lilaments, and many other uses will, no doubt, be found tor these filaments in the future because of Atheir inherent resistance to most solvents and other dependable properties of these materials.

lIlhe present invention relates to la process -ior the pro- .duction of microporous filaments or tibers, which are formed from polymers of l-olelins, and to the resulting microporous laments yand falticles formed therefrom.

'The porous laments obtained according to they invention have la softe-r texture Iand more orinkle than non-porous laments, thereby yielding `fabrics or other iilarnentous articles with different qualities and greater insulation properties than lilamentous arti-cles made from non-porous lillaments.

Accordingly, lan object of this invention is to provide novel microporous iilamentstormed from polymers. Y

Another object of this invention is to provide a method for preparing microporous lilaments or tibers formed from polymers. A further object of this invention is to provide l-oleiin polymer larnents having improved tent-ure, greater insulating properties, :and the like.

Other aspects, objects, as rwell as the several advantages of this invention -Will become readily apparent upon a study of this disclosure, the drawing and the appended claims. y According to the inventionfnovel microporous lilaments improved properties )are formed by blending together a l-olelin polymer land la styrene polymer, extrudi-ng the polymer blendl into one or more filaments, drawing a filament of said blend to elfect orientation of same, extracting the styrene polymer from` the `drawn iilament and thus form a microporous lilament, and recovering said porous iilarnent ras a product of the process. (When desired, the recovered microporous filament can 4be redraWn tor maximum tensile strength with la corresponding reduction in porosity, f

-As la further feature of the invention, an additional ,y plasticizer can be employed `along lwith the polystyrene during the formation of the 1-olet`1n polymer-polystyrene blend. .These additional plasticizers can be the high boiling esters Vincluding both inorganic and organic acid esters. 'Ille Iadded plasticizer is removed from the lila- Patented May 5, 1964 ICC ' ment along 'withvthe polystyrene during the extraction step. y

Tlhe polyoleiins or l-olefln polymers that can be `employed according to the invention include the solid `these `l-oleiins Vwith each other. YPolyoleiinspreferred .according to .the invention include polymers of ethylene and polymers of propylene. Specific representative examples of polyoletins that can be employed include polyethylene, polypropylene, ethylene-propylene copolymers, ethylene-butene-l copolymers, ethylenedpentene-1 copolymers and the like. Generally, in the case of the copolymers formed by cop olyrnerization of ethylene fand la second monomer, the amount of said second monomer present during formation of the copolymers will be less than about ten percent, irequently less than ve percent of the total monomers charged to the polymerization zone. Ethylene hornopolymers and copolymers of ethylene las describedabove preferred according to the invention have a density ranging from 0.930 to V0.990, ordinarily 0.940 to 0.970.

The polyolefins or l-oletin polymers employed according to the invention can be prepared -by any of the well known methods which lare usually employed in the preparation of these polymers. One well-known method xfor preparing polymers of l-oleiins` is the chromiumoxidecatalyzed polymerization described in the Hogan et al. patent, -U.S. 2,825,721 (1958). The polyolens prepared by Hog-an et al. are characterized by their high density and high crystal-linlty. Y

ln addition to the foregoing method for preparing the l-olelin polymers, especially polymers of ethylene or propylene, the invention Salso includes Within its scope the preparation of microporous lilaments prepared from polyolefms formed by polymerization in .the presence of an organometal catalyst such as a trialkylaluminum in conjunction With a titanium halide or salt. The polymers resulting from the chromium oxide catalyst polymerization according to Hogan et val., supra, lare preferred, and it is further preferred that the very highmolecular .Weight l-.oleiin polymers` which result from effecting the polymerization at a temperature below that which the polymer dissolves in the reaction diluent (also known as particle form polymer) .be used.

The Ipolystyrene which is employed in the formation ofthe blends according to the invention can .be a styrene homopolymer or one of the soy-called impact styrenes, these being blends of la major amount of polystyrene with a minor amount of rubber, generally of the butadienestyrene type. lt should tbe understood, liovvecer, that any of `the high or low molecular 'Weight polystyrenes las 'Well as the impact grades'y of this polymer can be employed. Any of the several commercially Aavailable polystyrenes can be employed in the invention and that polymer which is known in the las general purpose polystyrene is particularly suitable. The preparation of polystyrene is well known in the art and need not :be further discussed herein.

Fibers or monoiilaments that can be -formed into microporous articles according to the invention ordinarilyV have a diameter of 0.01 to mils (0.00001 to 0.1 inch), and more generally about 0.1 to 50 mils (0.0001 to I0.05 inch). The tensile strength of these libero cold drawn is above about 30,000 p.s.i., trequently about 90,000 to 100-,000 psi, and canrange las high as 150,000 to 250,000 p.s.i. measured at a temperature in the range of 65 to 100 Spun threads, `cordage rope, woven fabrics and other lilamentous 'articles can be prepared from the niicroporous tilaments of the invention. y

As indicated'above, plasticiz'ing agents such as the high boiling esters of both inorganic and organic acids can be incorporated into the l-oleiin polymer when blending sameA with a polystyrene.

Representative specific examples of plasticizers that can be used include the organic acid esters such as diisobutyl phthalate, diamyl phthalate, butyl octyl phthalate, butyl rdecyl phthalate, di-n-octyl phthalate, diisooctyl phthalate, butylphthalyl butyl glycollate, ethylphthalyl ethyl glycollate, methylphthalyl ethyl glycollate, diethyl phthalate, n-octyl n-decyl phthalate, glycerol phthalate, diphenyl phthalate, (di-tridecyl) phthalate, dibutyl adipate, di-n-hexyl adipate, dicapryl adipate, di(2-ethylhexyl) adipate, diisooctyl adipate, dinonyl adipate, octyl decyl adipate, dibenzyl adipate, diisooctyl Aazelate, diisobutyl azelate, butyl oleate, tetrahydrofurfuryl ioleate, n-butyl palmitate, isooctyl palrnitate, dibutyl sebacate, dibenzyl sebacate, octyl stearate, cyclohexyl stearate, glycerol stearate, and the like, inorganic acid esters such as tributyl phosphate, tri-(Z-ethylhexyD- phosphate, tri(butoxyethyl) phosphate, triphenyl phosphate, `diphenyl octyl phosphate, cresyl diphenyl phosphate, tricresyl phosphate, tri(dimet=hylphenyl)-phosphate, diphenyl o-xenyl phosphate, dibutyl butyl phosplronate, o,o,otriisooctyl phosphorothioate, and the like.

The amount of plasticizer incorporated into the polymer will generally range from about 5 to about 50 parts per 100 parts of polyolen/polystyrene blend when no ller is employed. (lf lillers are employed, the amount of plasticizrer can be as high as 100 parts per 100 parts of polymer blend. The preferred range `of plasticizer is from to about 40 parts per 100'` weight parts of polymer blend. The amount of iller or reinforcing agent employed will ordinarily range from about 1 to about 50 weight parts per 100 weight parts of polyolen, preferably 2-20 weight parts. Examples of suitable llers or rein- ;forcing agentsthat can be compounded into the polymer along with the plasticizer include clay, carbon black, silica, and the like. If pigments are to be used, only very small amounts are necessary to obtain intense coloration of the plasticized polymer. l

Iln forming the Yblends bf l-olefin polymer with polystyrene, the materials are blended together at a temperature above the softening point of each of these polymers. Generally, this blending step will be carried out at a ternperature between about 275 and 500 F., preferably from 300 to 450 F. 'Ihe specic blending temperature will depend upon the particular polymer being used and the particular type of equipment which is being used to form the blend. Conventional blending equipment such as Banbury mixers, roll mills and the like are quite satisfactory for blending these materials. In the subsequent step wherein a liber is extruded, a conventional type extruder with a filament die is required. A blending of the polymers in the extr-uder barrel can be carried out, if desired, although it may be necessary to employ an extended barrel on the extruder to obtain sufficient or adequate blending.

The amount of polystyrene which is blended with l-olelin polymer can vary over a wide range, but the polyolen/ polystyrene blend lwill generally contain from 5 to 50 weight percent polystyrene. Higher amounts can be used, but the ibers tend to go to pieces when subsequently extracted. Likewise, lower amounts can be used but one does not obtain any substantial degree of porosity in the ber.

Following the formation of the blends, the blended material is extruded into ilaments by means of conventional fiber or filament forming equipment. rIlhis can be carried out with -an extruder equipped with a lilament die, the die being either monoilament yor multilament. One of the advantages of the prese-nt invention is obtained in the extrusion step if one employs one ofthe very Vhigh molecular weight polymers which results from slurry polymerization. These polymers are extremely diicult to extrude or injection mold because of their very low melt index. In general, such polymers have an inherent viscosity in the range between 3.0 and 10.01, and ya high load melt index (ASTM DlZ38-57T-procedure F) in the range between 0.6 and 10.0. 'Ehe presence of the polystyrene in the blend serves as a processing aid and sharply increases its extrudab-ility.

A better understanding of the invention will be obtained upon reference to the accompanying drawing which diagrammatically illustrates one preferred embodiment of the invention in the formation of a microporous monofilament.

Referring now to the drawing, polyethylene by way of line 12 and polystyrene by way of line 11 are introduced into blending zone 10. Recycle polystyrene and plasticizer, when fused, as well as llers, dyes, etc., are introduced into blending zone 10 by way of lines 33, 34, 35 and 11. Blending of the polymers and additional added materials is eiected in zone 10 in a suitable blending apparatus, such `as a Banbury mixer, at a temperature above the softening point of thepolymers. However, as indicated above, blending of the polymers can be eiected -in an extruder when desired. The polyethylene-polystyrene blend obtained in zone 10 is removed by way of line 1'3 and introduced intoV extrusion zone 14 wherein the blend is forced through a die forming a strand 15. The extruded strand is passed through water-quench 16, which is usually maintained lat a temperature of about 75 F.

Quenched lament 15 is removed from quench 16, passed over roll 17, and then passed through drawing zone 19 having lrolls d8. The temperature for cold drawing is ordinarily in the range of about 100 to about 260 IF. The temperature is maintained by passing the filament through a bath of polyhydric alcohol such as glycerol while stretching the filament to five to ten times its origi- `nal length. The filament is drawn in zone 19 to orient the polyolelin molecules and obtain a product of usable properties, e.g., high tensile strength, etc.

The drawn tilament is removed from drawing zone 19, passed over roll 20 and then passed through extraction zone 2.2. having rolls 2l. Zone 22 contains a suitable solvent for the polystyrene in the lament as well as the plasticizer, when used. Zone 22 can be any apparatus adapted to provide contact of the lilament with the solvent under extraction conditions so as to remove the polystyrene and plasticizer from the iilament, thus leaving a porous l-oletn polymer lament. The preferred extracting agents employed in zone 22 are aromatic hydrocarbons such as benzene, xylenes, and similar aromatic materials. The extraction can be carried out at lany temperature `from approximately 50v to about 175 vF. 'Ilhis upper temperature is thel practical upper limit since the cloud point of the polyolen is approximately 180 F. in most hydrocarbon solvents. Generally, the yamount of solvent lwhich is used will be sutlicient to completely immerse the ilament in the solvent in zone *22. The time required for the extraction will vary over awide range but will generally range from about 5 minutes to about 100 hours, preferably 0.5 to 5 hours. Extraction zone 22 is sized and so constructed as to provide adequate contacting time of the nilament with the polystyrene solvent. As indicated above, the 'aromatic solvents are preferred since polystyrene is most soluble in these materials. However, any solvent for polystyrene which has little eiect on the polyolen at the above conditions can be used.

' Within zone 22, polystyrene `and any supplemental plasticizer employed are dissolved out of the filament,

thereby leaving a porous filament. The porous lilament is then removed from zone 22 and passed over roll 23 and then several alternative procedures can be followed.

As shown in the drawing, the porous monolament withdrawn from the extraction zone is first passed into a redrawing zone 25 having rolls 24. By redrawing the porous monofilament, a monoiilament having a much higher tensile strength is obtained. However, it should be recognized that the redrawing operation reduces the porosity of porous filament 15. Conditions of temperature and draw ratios employed in zone 25 can be substantially the'same as employed in zone 19. However,

ordinary redrawing is carried out at draw ratios ranging from 2 to 1 to 6 to 1, and higher, and temperatures ranging from about 100 to about 250 F. The redrawing operation offers an alternative procedure wherein antistatic agents or other materials can be incorporated into the porous monolament after etxraction and then the lilament redrawn to close the pores and trap the additive therein.

Alternatively, redrawn porous monofilament is Withdrawn from zone 25 and then either passed directly into storage drum 26 or passed through treating and drying zone 27 and thence to storage roll 26. Treating agents such as antistatic agents, antioxidants, ultraviolet `stabilizers and the llike can be incorporated into `the porouslament in zone 27 by introducing same by means of line 428. As indicated above, treating step 27 can precede the redrawing operationin zone 25. If desired,Y

fillers such as carbon black, graphite, titania, berglass and the like, as well as dyes can be introduced into the microporous filament, either before redrawing or after redrawing. However, in most cases, it is more convenient to introduce the fillers and dyes into lthe polymer before extrusion in blending zone 10. Surprisingly, the porous filaments ory fibers obtained according to the invention, after extraction, have more crinkle to them and thus have greater insulating properties when woven into fabrics. The bers also have a softer texture than unextracted non-porous bers.

Solvent, extracted polystyrene and supplemental plasticizer, if used, are removed from the base of zone 22 v by way of line 29 and passed to recovery zone 30. The

aromatic solvent is recovered in zone 30 and returned by way of line 31 to extraction zone 22. Recovery zone 30 can be fractionation, solvent extraction or other suitable separation systems known in the art. Polystyrene `and supplemental plasticizer, if used, are removed from zone 30 and returned to blending zone 10 by way of line 32,35 -and 11. Make-up plasticizer, if used, is introduced into recycle line 32 by way of line 34. Filler, for example, carbon black, is introduced into recycle line 32 by way of line 33. Fillers are preferably added to the polymer in the blending zone since they are not removed in the subsequent extraction step. Y As indicated above, the porous iilaments or fibers of the invention have much lower densities than the same shaped items from untreated polyolens.,` By Aoperating in' the mannerdescribed above, one can obtain the advantages of polystyrene asa processing aid during extrusion, yet by extracting and recycling the polystyrene one can use the polystyrene over and over againwith obvious economy. The extracted bers according to the invention contain practically` all of the tensile strengthof the unextracted fibers which is particularly surprising in View of the large amount of `material removed during the extraction step. Furthermore,the porouslarnents are well adapted for treating with agents such as antioxidants, antistatic agents and the like.

The following specific examples are intended to illustrate the advantages of the novel porous ibers or filaments Yof the invention and the process for their manufacture. However, it is `not intended to limit the invention to the embodiments shown in these examples as the examples merely illustrate the various advantages of the invention.

EXAMPLE I A blend o'f a high molecular weight polyolen (ethylene polymer) and polystyrene was formed, spun into a" liber, and extracted. Y

The high molecular Weight polyolen used in this run i Was an ethylene/ butene copolymer, prepared by. polyminsoluble (particle form poylmer) inthe reaction diluent. (Polymerization of ethylene-butene-l is carried out inl suspension and below temperature at which any substantial portion of the polymer formed isin solution in the suspending medium.) This polymer had the following properties:

The polystyrene used was' a commercial polystyrene manufactured by Koppers, sold under the tradename Dylene, having the following properties:

l Polystyrene Properties Density g`./cc 1.0724

Inherent viscosity 0.84 Hardness, Shore D 84 Tensile strength at yield p.s.i. 1188 The -above described polyolefin (ethylene-butene-l copolymer) was blended with the above described poly- Y styrene to form a blend containing 75% by Weight polyolefin and 25% by weight polystyrene. Blending was carried out in a heated Banbury mixer at 30G-370 F. The blend was vthen spun into a monolament lber and drawn at a draw ratio of 6.9 to 1. The initial diameter of the liber` was 0.040 inch, and the drawn ber diameter was 0.016 inch. The fiber was then soaked overnight in xylene at room temperature, after which the ber Was washed with hot acetone and hot chloroform to remove occluded xylene. Y After drying under vacuum, the polymerwas weighed, and it was found that the extraction had removed 20% of'the fiber weight. The extracted fiber was softer to the touch and more pliable than the unextracted fiber. The tensile strength of the fiber, before and after extraction, was as follows:

In another rim, a 90/10 polyolen (ethylene-butene-l copolymer) polystyrene blend was formed from the same polymers as above and extracted by the same method except that the extractions was carried out using" benzene at 160 F. instead of xylene at room temperature. The fiber properties (tensile) were as follows:

UnextractedV Extracted v.

ber l fiber Tensile at yield, p.s.i 55,536 49, 16s Elongation at break. percent `22 66 The Weight Vdecrease of the ber dueI to extraction was 9.4%.

EXAMPLE II In another run, porous bers were formedrby blending high molecular weight polyethylene, polystyrene and di-n-butylY phthalate, spinning the blend into bers, drawing the ber and subsequently extracting the thus-formed fibers.

In run, the polyethylene used was prepared by the polymerization of ethylene in the presence of a chromium oxide-containing catalyst at a temperature such that the polymer was insoluble in the polymerization medium. This polymer had the following properties:

Polyethylene Properties Volatiles Weight percent 0.0 Ashy -do---- 0.01 Melt index 1.87 Density g./cc 0.955 Izod impact strength No break Flexural modulus p.s.i. 209,000 Tensile (yield) do B835 Elongation percent 48 Environmental stress cracking,

time to fail hours 46 Inherent viscosity 3.27

The blend was formed by mixing together 80 parts by weight of the above described polyethylene, 10 parts by weight of polystyrene (identical to Example I) and l0 parts by weight di-n-butyl phthalate (DNBP) in a Banbury mixer at SOO-370 F. This blend was then spun into fibers and drawn at a draw ratio of 7.0-7.4/ 1.0. The ber was then extracted with benzene at 160 F., after which samples of the porous ber were redrawn by hand at 100 C. The extraction of the original fiber removed 18.1% of the fiber weight. The results of these runs are expressed below as Table I.

Fibers were formed of the blend described in Example II. These bers were extracted by the method of Example II after which the extracted ber was redrawn on the ber machine at a steam box temperature of 210 F. These runs are expressed below as Table II.

TABLE II Tensile Elonga- Fiber Diameter, (yield) tion at inches p.s.i. Break,

percent Unexnracted Fiber o. 012s 42, aso 103 Extracted Fiber. 0. 0120 49, 115 63 Redrawn Extracted I 0.0098 95, 480 21 D0 0.0100 88, 740 17 EXAMPLE IV In another series of runs, a lower molecular weight ethylene/butene-l copolymer was blended with polystyrene. This blend was extruded into a ber, after which the ber was extracted with benzene at 160 F. to remove the polystyrene. Y

The ethylene/ butene-l copolymer used in this blend was prepared by the solution copolymerization of ethylene and l-butene in the presence of a chromium oxide-containing catalyst. The copolymer had a melt index (ASTM Dl238-57T--Procedure E) of 0.03 and a density of 0.950 g./ cc.

. This copolymer was blended with the polystyrene described in Example I to form an 85/ 15 ethylene copolymer/polystyrene blend. The blend was formed by milling the polymer together at SOO-370 F. in a Banbury mixer.

The blend was then extruded into fibers, and a portion of 'the fiber was extracted with benzene at 160 F. to remove polystyrene. The extraction removed 14% of the polymer weight. The results of the ber tests are expressed below as Table III. Included in the table are properties of a ber formed from the ethylene/butene copolymer with no polystyrene present.

The extracted fiber from Table III was then redrawn on the fiber machine at a draw ratio of about 2 to 1. The results of these tests are expressed below as Table IV.

TABLE IV Steam Fiber Elongation Run Box Temp. Diameter, Tensile at at break, at Paw, Inches yield, p.s.i. percent EXAMPLE V In a further series of testing a blend of polypropylene and polystyrene was formed and extruded into multiiilament ber. The fiber was then extracted'with benzene at 160- F. to remove polystyrene.

First, parts by weight polypropylene (Profax made by Hercules-Type 6512-E-comrnercial extrusion polymer) and 15 parts by weight of the polystyrene described in Example I were blended together at 30G-370 F. in

a Banbury mixer. The polypropylene polymer employed had the following properties.

Tensile strength 4990 p.s.i.

Elongation 18%.

Shore D hardness 75.

Flexural modulus 213,000 p.s.i.

Density 0.9066.

Melt index 2.8 (measured at 230 C.).

The blend was then'extruded into multiilament ber through a 15 hole die. The bers were then drawn at 4/ 1 and 6/ 1 draw ratios. Portions of these drawn 'bers were then extracted for three hours with boiling benzene, after which the bers were dried overnight. Properties of'these libers are expressed below as Table V.

In still another run, grams of 65/ 35 blend of the ethylene-butene-l copolymer ofExample I/polystyrene was blended with 35 grams of di-n-butyl phthalate and 9 molded and extracted `with benzene at 113 F. for eight hours. The slabs formed lwere 1/s" x 5% x 51/8. Buftiug of the slabs with silk or cotton caused a buildup of static charge of 3.5 to 9.75 kilovolts. One of the slabs was treated with a methanol solution of a commercial antistatic agent suicient to provide01 weight percent of the agent based on the polymer. The agent used was Al- Y rosperse 1l-P, an agent sold by Geigy Chemical. Analysis of this agent showed it to be a compound of the formula t i R-b-N-CHi-CHZ-OH t CH2-Curon wherein R is a 12 to 14 carbon alkyl radical. Testing of the treated slab for static by buing followed by contact with a voltmeter showed no static buildup. However, in

four days, static tests showed a buildup of 7 kilovolts. When the porous polymer was treated with a methanol solution of the agent sulicient to provide 0.6 Weight percent of the agent based on the polymer, no static buildup could be eliected over a test period of 200 days.

EXAMPLE VII In still another run, a blend of 50 parts of the ethylene/ butene copolymer (particle form polymer) of the preceding examples, 50 parts of the polystyrene of the preceding examples and 20 parts of comminuted glass` wool was prepared by the procedure of the preceding examples. Molding of a slab at 320 F. and extraction with benzene at 160 F. for four hours produced a porous polymer containing dispersed glass ber.

Similar runs were carried out using carbon black and TiOz as fillers. Porous polymers containing from to 15 percent carbon black were formed, as were porous polymers containing `similar levels of titania. Surprisingly, the carbon black did not extract out when the polystyrene was removed by extraction with hot benzene. Y

The test procedures employed in the preceding examples are set forth below.

Melt index-:ASTM D123t8-57T-Procedure 'F unless noted otherwise.

Density=ASTM D1505-60T. Density is run on acompression molded sample, molded at 325 for ve minutes and cooled at rate of 27i4 F./minute down to approximately 150 F. (where it can be handled).

Izod impact strength=ASTM D256-56, Method A.

Flexural modulus=ASTM D790-58T.

Tensile and elongation=ASTM D63 8=5 8T.

Environmental stress cracking=ASTM Dl693-60T.

Inherent viscositydetermined by dissolving an exact amount of polymer (amount will be chosen between 0.02 and 0.1000 gram) in 100 ml. of tetralin at 130 C. The inherent viscosity is calculated as the dow time inseconds in tetralin, both ilovv times being measured on a size 50 Ostwald-Fenskeviscosimeter. From the viscosity, molecular weight can be calculated by spirit and scope of the invention.

We claim:` f

l. A process for the production of a porous filament which comprises the steps of blending a l-oleiin polymer with from 5 to 50 weight percent of a styrene polymer, shaping said blend into a iilament, drawing the filament formed from said blend to orient same, and removing a substantial portion of said styrene polymer from said ftlament by contacting same with an aromatic hydrocarbon solvent at a temperature ranging from about 50 F. to about the cloud point or said l-olefin polymer, thus leaving a porous iilament of said l-olelin polymer.

2. A process for the production of a porous filament which comprises the steps of forming a blend of a l-oleiin polymer and polystyrene containing from 5 to 50 weight percent polystyrene, extruding said blend into a filament, drawing said filament formed from said blend within a draw ratio ranging from 2 to l to 10` to 1 to eiect orientation of same, contacting said drawn iilament with an aromatic hydrocarbon solvent under extraction conditions Iat a temperature ranging from about 50 to about 175 F. `and a period of time suiicient to remove a substantial proportion of said polystyrene therefrom, thus forming a porous filament of said l-olein polymer, and recovering said porous filament as a product of the process.

3. A process 4according to claim 2 wherein said porous filament is rednawn within a draw ratio ranging from 2 to l to 6 to 1 to form `a iilament of maximum tensile strength with a corresponding decrease in porosity.

4. The process of claim 2 wherein a high boiling ester is also incorporated into said blend and which is also removed along with said polystyrene during said extraction so as Ito incre-ase the porosity of said filament.

5. A process for the production of a microporous lament of maximum tensile strength which comprises the steps of blending together a l-olen polymer and polyextract said polystyrene therefrom and form a porous monoilament, recovering polystyrene extracted during said contacting from said solvent and recycling same to said blending, and recovering said porous filament asa product of the process.

6. A process according to claim 5 wherein said olen polymer is an ethylene polymer and wherein an organic acid ester is also incorporated into said blend and subsequently extracted along with said polystyrene to increase the porosity of said filament.

7. F[The process of claim v5 wherein said recovered porous lilament is treated with an antistatic agent and then redrawn within a draw ratio of 2:1 to 6:1 to form a ilament of maximum tensile strength with a corresponding decrease in porosity, thereby confining a portion of the antistatic agent Within the pores of said lament.

References Cited in the le of this patent p UNITED STATES PATENTS 

1. A PROCESS FOR THE PRODUCTION OF A POROUS FILAMENT WHICH COMPRISES THE STEPS OF BLENDING A 1-OLEFIN POLYMER WITH FROM 5 TO 50 WEIGHT PERCENT OF A STYRENE POLYMER, SHAPING SAID BLEND INTO A FILAMENT, DRAWING THE FILAMENT FORMED FROM SAID BLEND TO ORIENT SAME, AND REMOVING A SUBSTANTIAL PORITON OF SAID STYRENE POLYMER FROM SAID FILAMENT BY CONTACTING SAME WITH AN AROMATIC HYDROCARBON SOLVENT AT A TEMPERATURE RANGING FROM ABOUT 40*F. TO ABOUT THE CLOUD POINT OF SAID 1-OLEFIN POLYMER, THUS LEAVING A POROUS FILAMENT OF SAID 1-OLEFIN POLYMER. 